An exemplary turbofan gas turbine engine for powering an aircraft includes in serial flow communication a fan, a compressor, a combustor, a high pressure turbine (HPT) which powers the compressor, a low pressure turbine (LPT) which powers the fan, an afterburner, and a variable area exhaust nozzle (VEN). The fan includes a plurality of circumferentially spaced apart fan blades extending radially outwardly from a rotor which are effective for compressing air received by the engine during operation. The fan has a conventional performance map indicating its ability to provide pressure increase of the inlet airflow during various operations of the engine while maintaining a suitable stall margin. Stall margin is a conventional parameter for ensuring the prevention of fan stall as the engine is operated in a conventional flight envelope of the aircraft being powered by the engine.
The engine further includes a conventional control system including several subsystems thereof which control the various components of the engine. For example, a VEN control subsystem controls actuators which in turn control the discharge flow area of the VEN as thrust generated by the exhaust gases discharged therefrom is varied. The area of the VEN is conventionally selectively opened, or increased, and closed, or decreased, as required during operation. This is typically done by providing within the overall engine control system a predetermined VEN schedule which provides a suitable area demand signal to the VEN control subsystem for suitably positioning the VEN.
This exemplary engine also includes conventional variable guide vanes (VGVs) associated with the fan for selectively controlling the inlet airflow therethrough. A VGV control subsystem conventionally controls the angular position of the vanes between open and closed positions thereof which provide predetermined maximum and minimum, respectively, flow area through the vanes for regulating the inlet airflow to the fan. The engine control system additionally includes a predetermined VGV schedule to control the opening and closing of the VGVs during operation of the engine.
It is conventionally known to vary the position of the VGVs and the area of the VEN, among other parameters, to control stall margin of the fan. However, in a derivative engine being developed, the fan performance map includes a pinch point region wherein the stall margin decreases to less than the desired value thereof during operation of the engine over the portion of the flight envelope and in a transient condition known as throttle chop during which the fan abruptly decelerates. Accordingly, it is desired to increase the fan stall margin to reduce the pinch point region while using the conventional structures of the engine to minimize required hardware changes.